What should you look for in a ski boot? Firstly, it must be comfortable: you will be wearing it for around seven hours a day when you're skiing.
Secondly, the boot must form an effective link between you and the snow. It must be sufficiently rigid laterally to transmit movements of your legs into ski movements, but must still allow some forward flex at the ankle.
Finally, it must hold your foot – especially your heel – firmly in place.
The basic construction of all ski boots is similar, consisting of a rigid polyurethane shell, with a soft removable inner. Many top end boot inners and shells can now be heat moulded to your foot, making them even more comfortable.
Actual experience varies from boot to boot and from foot to foot, so make sure you try on a few.